Fluid Turbine With Turbine Shroud And Ejector Shroud Coupled With High Thrust-Coefficient Rotor

ABSTRACT

The present disclosure includes a shrouded fluid turbine that generates large wake expansion to generate power coefficients greater than the Betz power coefficient of 16/27 based on maximum system area. The Virtual Diffuser Wind Turbine (VDWT) system operates differently with, and without power extraction. With power extraction the VDWT system includes a high rotor thrust coefficient combined with an annular shroud system that uses bypass flow to form an annular, high-energy bypass flow stream that flows outward at the exit plane of the VDWT generating an observable virtual diffuser downstream therefrom. This outward, high-energy flow reduces exit plane pressure generating large wake expansions. The large wake expansion increases the flow rate through the rotor. Without rotor power extraction, separation takes place which reduces some of aerodynamic loads on the VDWT at extreme winds.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 61/980,240, filed on Apr. 16, 2014, the disclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

The present disclosure relates to a fluid turbine system having a rotor coupled with an electrical generator and, more particularly, to increased turbine performance from the combination of an ejector duct system and a rotor designed to allow a high rotor thrust coefficient by using a Coanda bypass stream to increase downstream wake expansion.

In general, some conventional horizontal axis fluid turbines used for power generation have blades (e.g., two to five open blades) arranged like a propeller, the blades often being coupled to a power generator by an energy transfer means.

Generally speaking, a fluid turbine structure with an open, for example, unshrouded rotor design captures energy from a fluid stream that is smaller in diameter than the rotor. In an open unshrouded rotor fluid turbine, as fluid flows from the upstream side of the rotor to the downstream side, the average axial fluid velocity remains constant as the flow passes through the rotor plane. Energy is extracted at the rotor resulting in a pressure drop on the downstream side of the rotor. The fluid directly downstream of the rotor is at sub-atmospheric pressure due to the energy extraction and the fluid directly upstream of the rotor is at greater than atmospheric pressure. The high pressure upstream of the rotor deflects some of the upstream air around the rotor. In other words, a portion of the fluid stream is diverted around the open rotor as if by an impediment. As it is diverted around the open rotor, the fluid stream expands, which is referred to as flow expansion at the rotor. Due to the flow expansion, the upstream area of the fluid flow is smaller than the area of the rotor.

In contrast, in a ducted or shrouded turbine, the upstream area of the fluid stream can be larger than the area of the rotor. The duct or shroud contracts the fluid stream at the rotor plane and the fluid stream expands after leaving the duct or shroud. The energy that may be harvested from the fluid is proportional to the upstream area where the fluid stream starts in a non-contracted state.

In the field of fluid energy conversion, turbines are typically mounted on vertical support structures at the approximate center of gravity of the turbine and near the center of pressure. The center of pressure is the point on the turbine where all of the aerodynamic pressure field may be represented by a single force vector with no moment. The center of pressure of a turbine is typically near the downwind portion of the rotor plane. The point at which the support structure engages the turbine is often behind the rotor plane at the nacelle. A support structure engaged with a turbine, up-stream from the rotor is referred to as a downwind turbine.

The Betz limit calculates the maximum power that can be extracted from a volume of moving fluid by an open blade, horizontal axial flow turbine (HAWT). The Betz limit is derived from a fluid dynamic control-volume analysis for flow passing through an open rotor, and by applying one-dimensional equations based on the principles of the conservation of mass, momentum and energy. According to the Betz limit, and independent of the design of the fluid turbine, a maximum of 16/27 of the total kinetic energy in a volume of moving fluid can be captured by an open-rotor turbine. Conventional turbines commonly produce 75% to 80% of the Betz limit.

A fluid turbine power coefficient (Cp) is the power generated over the ideal power available by extracting all the wind kinetic energy approaching the rotor area. The Betz power coefficient of 16/27 is the maximum power generation possible based on the kinetic energy of the flow approaching the rotor, and the rotor area. For an open rotor, the rotor area used in the Betz Cp derivation is the system maximum flow area described by the diameter of the rotor blades. The maximum power generation occurs when the rotor flow velocity is the average of the upstream and downstream velocity. This is the only rotor velocity that allows the flow-field to be reversible, and the power extraction to be a maximum. At this operating point, the rotor velocity is ⅔ the wind velocity, the wake velocity is ⅓ the wind velocity, and the rotor flow has a non-dimensional pressure coefficient of −⅓ at the rotor exit. The −⅓ pressure coefficient is a result of the rotor wake flow expanding out to twice the rotor exit area downstream of the rotor station.

There has been considerable effort and discussions concerning the potential for ducted fluid turbines to outperform their non-ducted counterparts (i.e., to surpass the theoretical power extraction limit defined by Betz limit). All of these discussions use the Betz limit based on rotor area where the rotor area is significantly smaller than the ducted wind turbine maximum area. Results presented by Dr. Gerard J. W. van Bussel in “The Science of Making More Torque From Wind: Diffuser Experiments and Theory Revisited” published in the Journal of Physics: Conference Series 75, 1-12 (2007) demonstrate that no one has yet surpassed the Betz limit based on system maximum area.

When a column of fluid encounters a working rotor it is an impediment. Therefore, a portion of the fluid approaching the rotor flows around, and not through the rotor. Fluid flowing around the rotor is referred to as bypass flow. Bypass flow passes over the outer surface of the flow-stream tube and does not pass through the rotor. As more energy is extracted at the rotor, a blockage effect increases. In other words, as more power is extracted at the rotor, more flow bypasses the rotor. The power rating of a wind turbine results from balancing these opposing effects.

A fluid turbine power coefficient (Cp) is a function of wake velocity ratio and thrust coefficient (Ct). Thrust coefficient is the ratio of pressure drop across the rotor over the dynamic pressure of the fluid flow and represents an important parameter for the design of the rotor. The turbine exit plane pressure coefficient, (Cpe), is the fluid turbine system exit plane pressure difference over the free stream dynamic pressure. Exit plane pressure coefficient can be used to define wake expansion.

SUMMARY

The present disclosure teaches systems and methods to exceed the Betz limit based on maximizing wake area expansion by using rotor power extraction to favorably modify the shroud flow for increased power generation. The present disclosure is based on using power extraction to enhance downstream wake expansion generated by a shrouded fluid turbine system, and therefore increasing maximum Cp.

In some embodiments a fluid turbine system is disclosed. The fluid turbine includes a plurality of blades mounted to a rotor assembly. The rotor assembly has an operating thrust coefficient sufficient to create a flow region downstream of the rotor assembly that has a significantly lower energy level than a primary fluid flow.

The fluid turbine includes a first annular duct having an annular leading edge, an annular trailing edge, an inner surface extending between the leading edge and the trailing edge, an outer surface extending between the leading edge and the trailing edge and a central axis. The inner surface of the annular duct is in fluid communication with the plurality of blades.

The fluid turbine includes a second annular duct having a leading edge, a trailing edge, an inner surface extending between the leading edge and the trailing edge, an outer surface extending between the leading edge and the trailing edge, and a central axis. The inner surface of the second annular duct in fluid communication with the outer surface of the first annular duct.

The fluid turbine includes a Coanda nozzle formed between the outer surface of the first annular duct in proximity to the leading edge of the second annular duct and the inner surface of the second annular duct in proximity to the leading edge of the second annular duct. The Coanda nozzle is capable of forming a high-energy bypass fluid flow stream between the outer surface of the first annular duct and the inner surface of the second annular duct that flows along the inner surface of the second annular duct when power is extracted from the primary fluid flow by the rotor assembly. The high-energy bypass fluid flow stream flows outwardly from the turbine system exit plane forming an observable aerodynamic virtual diffuser downstream of the exit plane of the second annular duct.

In some embodiments a method of increasing energy extraction from a fluid stream is performed. The method includes forming a high-energy bypass flow between an annular gap formed between an outer surface of a first annular duct and an inner surface of a second annular duct downstream of the first annular duct. The method includes forming an observable aerodynamic virtual diffuser from the high-energy bypass flow downstream of an exit plane of the second annular duct and forming, with the low exit plane pressure provided from the high-energy bypass flow, a wake area larger than an area defined by an exit plane of the second annular duct. The method includes extracting energy from the flow stream with a rotor assembly having a thrust coefficient of between 0.89 and 1.0.

In some embodiments, a ducted fluid turbine is disclosed. The ducted fluid turbine includes a first annular ducted shroud and a second annular ducted shroud disposed downstream of the first annular ducted shroud. An annular gap is formed between a top trailing edge surface of the first annular shroud and a leading edge of the second annular shroud. The annular gap has a height sufficient to form a fluid jet along an inner surface of the second annular shroud to form an observable aerodynamic virtual diffuser downstream of an exit plane of the second annular shroud. A rotor assembly is disposed in the first annular shroud. The rotor assembly has a thrust coefficient of between 0.89 and 1.0.

These and other non-limiting features or characteristics of the present disclosure will be further described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which are presented for the purposes of illustrating the disclosure set forth herein and not for the purposes of limiting the same. Example embodiments of the present disclosure are further described with reference to the appended figures. It is to be noted that the various features and combinations of features described below and illustrated in the figures can be arranged and organized differently to result in embodiments which are still within the spirit and scope of the present disclosure. To assist those of ordinary skill in the art in making and using the disclosed systems, assemblies and methods, reference is made to the appended figures, wherein:

FIG. 1 is a schematic of an example prior art shrouded fluid turbine system;

FIG. 2 is a graph depicting the predicted power coefficient based on rotor area of a prior art fluid turbine system;

FIG. 3 is a schematic of an example prior art Diffuser Augmented Wind Turbine (DAWT) system;

FIG. 4 is a graph depicting the power coefficient for a prior art DAWT system on the vertical axis over the wake to wind velocity ratio on the horizontal axis referenced by rotor area;

FIG. 5 is a graph depicting the power coefficient for a prior art DAWT system on the vertical axis over the wake to wind velocity ratio on the horizontal axis referenced by shroud exit area;

FIG. 6 is a schematic of an example prior art shrouded fluid turbine system illustrating conventional wake expansion downstream of the system exit plane;

FIG. 7 is a graph depicting the power coefficient of the prior art turbine system depicted in FIG. 6 over the thrust coefficient of the rotor of the system for a number of turbine examples with different wake expansions reflected by negative exit plane pressure coefficients;

FIG. 8 is a diagrammatic cross section depicting Coanda flow effect;

FIG. 9 is a diagrammatic cross section of an embodiment of a VDWT as taught herein;

FIG. 10 is a diagrammatic cross section depicting modeled flow field through an embodiment of a VDWT as taught herein;

FIG. 11 is a detailed view of a modeled aerodynamic virtual diffuser of the diagram of FIG. 9;

FIG. 12 is a cross sectional view of an embodiment of a VDWT as taught herein depicting a second mode of operation absent an aerodynamic virtual diffuser;

FIG. 13 is a diagrammatic cross section of another embodiment of a VDWT as taught herein;

FIG. 14 is a diagrammatic cross section of another embodiment of a VDWT as taught herein;

FIG. 15 is a front perspective view of an example embodiment of a VDWT as taught herein;

FIG. 16 is a rear perspective view of an embodiment of FIG. 15;

FIG. 17 is a front perspective view of an example faceted embodiment of a VDWT as taught herein of the present disclosure having pivotable faceted segments;

FIG. 18 is a rear perspective view of an the embodiment of FIG. 17;

FIG. 19 is a detailed cross sectional view of an embodiment of FIG. 17;

FIG. 20 is a front, perspective, and detailed view of an embodiment of FIG. 17 showing primary airfoil segments pitched toward a stream line;

FIG. 21 is a front, perspective, and detailed view of an embodiment of FIG. 17 showing primary and secondary airfoil segments pitched toward a stream line.

FIG. 22 is a cross sectional, orthographic, detail section view of an embodiment of FIG. 17 illustrating the pitched ejector airfoil segments pitched at a stream line;

FIG. 23 is a front perspective view of an example embodiment of a VDWT having more than two shrouds with the rotor in front of the first shroud as taught herein.

FIG. 24 is a front perspective view of an example embodiment of a VDWT having more than two shrouds with the rotor inside the first shroud as taught herein.

DETAILED DESCRIPTION

The present disclosure teaches a Virtual Diffuser Wind Turbine (VDWT) system. The VDWT system includes a rotor designed with a high thrust coefficient combined with an annular duct system that uses bypass flow to form an annular, high-energy Coanda flow at the system exit plane. This annulus of high-energy Coanda flow (high-energy diffuser flow) creates an extremely low exit plane pressure, resulting in a large wake expansion. As such, the rotor power extraction is used to enhance downstream wake expansion, and therefore increase maximum Cp as taught herein.

The VDWT system combines increased wake expansion with a high thrust coefficient rotor to operate at power coefficients greater than 16/27 based on maximum system area. The same VDWT system can operate efficiently at lower thrust and power coefficients. The VDWT system also operates differently with and without power extraction. That is, the VDWT system is a bimodal wind turbine. When power is extracted, bypass flow is used to generate a Coanda stream that results in large wake expansion and power production. Without rotor power extraction, flow separation occurs in the system. The flow separation decreases wake expansion and lowers the flow rate through the system lowering pressure loads at extreme winds.

The VDWT systems taught herein are capable of exceeding the Betz limit. In order to do so, the VDWT systems taught herein have rotor-thrust coefficients (Ct) between 0.89 and 1.0 in combination with a large wake expansion. When these conditions are met, power production of a VDWT system as taught herein can exceed the Betz limit based on maximum shroud area. Based on computational fluid dynamics modeling of a VDWT, potential performance coefficients between 0.592 and 0.8 are predicted as driven by the high Ct, low Cpe, and increased wake expansion.

Conventional understanding teaches that the increased performance of diffuser augmented turbines is due to higher flow velocities at the rotor plane. The present disclosure teaches away from the conventional understanding and shows that the performance of conventional shrouded fluid turbines with or without duct diffusion is limited to the Betz limit based on maximum flow area when there is no wake expansion downstream of the system exit plane. The present disclosure further demonstrates that increased rotor velocities resulting from increased flow diffusion in the shroud system do not allow the system to exceed the Betz limit based on maximum system area. As taught herein, generating significant wake expansion downstream of a shroud exit plane via low exit pressure enhanced by a high energy Coanda stream does allow a shrouded fluid turbine to exceed the Betz limit.

As taught herein a VDWT system is a horizontal-axis fluid turbine having a turbine and ejector shroud aligned and spaced apart to form one or more Coanda annular flow nozzles between the two shrouds when rotor power is extracted. Coanda flow occurs when a stream of high-energy flow is injected parallel to a curved surface in contact with a large, low velocity fluid. With the Coanda annular flow nozzle or nozzles, the bypass flow is the high-energy flow, and the rotor power extraction is designed to provide the low velocity fluid. The high-energy flow stays attached to the curved ejector surface even over large outward angular deflections because there does not exist a sufficiently large adverse pressure gradient. The VDWT high-energy stream is directed outwardly away from an ejector shroud at the exit plane by the shroud contouring. The high-energy bypass flow allows for low exit plane pressures which generates a large wake expansion. The high energy flow bounds the low-velocity flow in such a way that it forms an observable aerodynamic surface, for example, an observable virtual diffuser surface. However, the mechanism for the wake expansion is the low exit plane pressure. In turn, the virtual diffuser coupled with a highly loaded rotor design provides increased power generation compared to open rotor and other shrouded fluid turbines. When no power is extracted, the system stalls producing substantially lower velocity at the rotor plane.

Unlike conventional Diffuser Augmented Wind Turbines (DAWT) systems, the bypass flow in the VDWT is not employed to energize the diffuser boundary layer. Boundary layer slots have been used extensively in DAWT's to increase shrouded wind turbine performance with little success. Employing bypass flow to energize the boundary layer over an annular airfoil surface commonly employs relatively small slot heights (near the thickness of the wall boundary layers), different placement, different slot designs and different diffuser designs than those described in the present disclosure. Slots for energizing diffuser boundary layers require a height that is less than 1% of the shroud length, and a flow rate similar to the boundary layer flow rate. These small slots have to be placed in the adverse gradient region of the diffuser slightly before the separation location, and the slot shape design typically has little wall curvature. The injected flow is designed to energize the thin, wall boundary layer. Results presented by Dr. Gerard J. W. van Bussel in “The Science of Making More Torque From Wind: Diffuser Experiments and Theory Revisited” published in the Journal of Physics: Conference Series 75, 1-12 (2007) demonstrate that even with boundary layer slots, no one has yet surpassed the Betz limit based on system maximum area.

The bypass duct annulus on an example VDWT has a height greater than 5% of the shroud length and often can vary between 5% and 25%. In some embodiments, a slot height of about 10% of the shroud length and flow around 20% of the system total flow rate. The slot is positioned in an accelerating flow location of the shroud, and is contoured to form a Coanda nozzle along the ejector surface. In accordance with the teachings herein, the high-energy bypass flow is designed to pass over the inner surface of the ejector and provides significant outward momentum at the system exit plane to form the observable virtual diffuser, rather than just locally energizing the diffuser boundary layer.

High-energy Coanda flow follows the inner surface of the ejector and turns to substantially large outward angles (20 degrees to 60 degrees relative to the central axis) at the ejector exit plane. This outward Coanda-flow momentum at the exit plane is able to lower exit plane pressure, allowing for further wake expansion.

Advantageously, the structure, operation and function of a VDWT as taught herein allows for bimodal operation. That is, any ducted or shrouded turbine has a mass and surface area that can exhibit a structural challenge, particularly in high velocity, extreme wind conditions. Often, wind turbines are designed to withstand extreme wind conditions but not to produce power. Without rotor power-extraction flow separates over the shroud surfaces resulting in reduced flow through the turbine, which in turn provides reduced aerodynamic pressure loads in high wind conditions.

As taught herein, at least one annular airfoil is in fluid communication with the circumference of a rotor plane. The structure of the VDWT system, as described herein, also allows a shroud structure where the turbine shroud is significantly shorter than the ejector shroud. In some embodiments the VDWT system has a turbine shroud that is 30% to 70% shorter than the ejector shroud, and contains a significantly higher diffuser area ratio.

Some embodiments as taught herein include annular airfoils with faceted segments. The faceted VDWT embodiments can also be used to decrease drag. Movable, faceted segments for the longer ejector shroud design may be moved to reduce diffusion and therefore significantly reduce extreme wind drag values. Some embodiments include articulated portions for example, at least one airfoil element. Such an articulated airfoil element may include a linear-stacking constant cross-sectional segment rotatable about an axis that is substantially perpendicular to the turbine central axis. One skilled in the art will understand that such linear-stacking, constant cross-sectional segments may be passively, mechanically, or pneumatically actuated.

Some embodiments are configured such that the leading edge of the ringed airfoil, or turbine shroud, is tangent to, or down-stream of, the rotor plane. A duct or shroud including a ringed airfoil provides a lift coefficient on the inner surface of the airfoil. Multiple element airfoils increase the camber of an airfoil and raise the maximum lift coefficient. Increased lift coefficient of a ringed airfoil surrounding a turbine reduces the minimum fluid velocity at which the turbine is able to operate, otherwise referred to as a reduction in the cut-in speed of the turbine.

As understood by one skilled in the art, the aerodynamic principles presented in this disclosure are not restricted to a specific fluid, and may apply to any fluid, defined as any liquid, gas or combination thereof, and therefore includes water as well as air. In other words, the aerodynamic principles of a VDWT system in air apply to hydrodynamic principles in a VDWT system designed for water.

The example embodiments disclosed herein are illustrative of shrouded turbine systems that advantageously direct a high-energy bypass flow to allow for downstream wake expansion bounded by an observable aerodynamic surface or virtual diffuser. It should be understood, however, that the disclosed embodiments are merely examples of the present disclosure, which may be embodied in various forms. Therefore, details disclosed herein with reference to example fluid turbine systems or fabrication methods and associated processes or techniques of assembly and or use are not to be interpreted as limiting, but merely as the basis for teaching one skilled in the art how to make and use the advantageous fluid turbine systems of the present disclosure.

A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying figures. These figures are intended to demonstrate the present disclosure and are not intended to show relative sizes and dimensions or to limit the scope of the example embodiments.

Although specific terms are used in the following description, these terms are intended to refer to particular structures in the drawings and are not intended to limit the scope of the present disclosure. It is to be understood that like numeric designations refer to components of like function.

The term “rotor” or “rotor assembly” is used herein to refer to any assembly in which blades are attached to a shaft and able to rotate, allowing for the generation of power or energy from fluid rotating the blades. Example embodiments of the present disclosure depict a fixed-blade rotor or a rotor assembly having blades that do not change configuration so as to alter their angle or attack, or pitch. Example rotor assemblies of the present disclosure can also include a variable pitch blade rotor, a propeller-like rotor, or a rotor or stator assembly. Any type of rotor assembly may be utilized with the fluid turbine systems (e.g., shrouded fluid turbine systems) of the present disclosure.

As used herein, “rotor flow” or “rotor fluid flow” refers to a primary fluid flow that flows into an inlet of a first shroud, first annular duct or turbine shroud.

As used herein, “bypass flow” refers to a secondary fluid flow that flows past an outer surface of a first shroud, first annular duct or turbine shroud.

As used herein, “duct” or “shroud” refers to an airfoil ring having a circular cross section, an oval cross section, or a polygonal cross section and can be have a continuous inner and outer surface free of gaps or can have inner and outer surface portions interspaced with gaps, slots or holes.

As used herein, “observable virtual diffuser” or “observable aerodynamic surface” or “observable aerodynamic virtual diffuser” refers to a fluid stream formed by the fluid turbine that appears as a diffuser for the low energy flow downstream of an exit plane of the fluid turbine. When formed the fluid stream may be observed or the effect thereof may be observed or both.

In certain embodiments, the leading edge of a turbine shroud assembly may be considered the front of the fluid turbine system, and the trailing edge of a turbine shroud assembly or of an ejector shroud assembly may be considered the rear of the fluid turbine system. A first component of the fluid turbine system located closer to the front of the turbine system may be considered “upstream” of a second component located closer to the rear of the turbine system. Put another way, the second component is “downstream” of the first component.

Coanda flow is a fluid flow concept that produces a higher propulsion force on a propelling surface. A thin stream of high-energy flow (Coanda flow), when injected parallel to a curved surface in contact with a large, low energy or stationary fluid, stays attached and follows the curved surface even over large angular deflections. With respect to the shrouded fluid turbines taught herein, Coanda flow is seen to further reduce the exit plane pressure that draws the low energy flow from the rotor, in the same direction that the Coanda flow is moving.

Coanda flow can occur when: a high-energy flow is present, a low energy medium is present, a curved surface is present, and a nozzle designed to inject the high-energy flow into the low energy fluid medium and parallel to the curved surface is present. When all factors are present, a high-energy, Coanda jet is injected along the leading edge of the ringed airfoil surface. The Coanda jet retains high-energy and stays attached to the surface over large turning angles.

FIG. 1 is a diagrammatic cross section that presents an example of a prior art shrouded fluid turbine having a constant flow area from inlet to exit. A shrouded fluid turbine 100 includes a rotor 140 that rotates about a centerline 105 and is coaxial with an annular airfoil 112. The annular airfoil 112 has an inlet 117 and an outlet or exit (egress) plane 129. A control flow volume 133 is assumed straight and parallel to the centerline 105 of the annular airfoil 112 of the shrouded fluid turbine 100. The shroud and rotor forces on the moving fluid are represented as 118 and 120. The upstream wind flow velocity is depicted as 135. Rotor flow 122 is that which enters the rotor 140 in a conical form within the boundaries depicted by lines 116. Rotor flow 122 that has passed through the rotor 140 and through the exit plane 129 forms exit flow 124/128. The streamlines 124/128 at the shroud exit plane 129 are assumed straight and parallel. This assumption results in ambient pressure at the shroud exit plane 129, since the pressure has to be constant in the vertical direction. Streamlines 124 represent flow that bypasses, or flows around the shroud.

Streamlines 128 represent low energy flow that passes through the rotor. The dashed line 133 defines the control volume used to analyze system power generation using flow conservation principles. The shroud and rotor forces on the moving fluid are represented by 118 and 120. The mass flow 130 exiting the control volume 133 is a result of turbine system blockage. Power 131 is extracted by the rotor. The wake velocity 128 is the same value as the rotor velocity for the control volume specified 133. The wake velocity ratio, Vwake/Vwind, represents the wake velocity 128 over the oncoming wind velocity 135. The rotor flow 122 goes through no diffusion and no wake expansion downstream of the rotor exit 129.

FIG. 2 presents a graph plotting the predicted possible power coefficient 136 as a function of wake to wind velocity ratio (Vwake/Vwind) 138 of a prior art shrouded fluid turbine. The maximum power coefficient occurs when the wake velocity is about 1/√{square root over (3)} of the velocity of the oncoming fluid stream 135. The maximum power coefficient Cp 134, is about 0.385 which is significantly below the Betz coefficient value of 16/27 for an open rotor. The results show that an open flow rotor has much higher energy potential when compared to a prior art shrouded fluid turbine. Comparing the results in detail, it is seen that this higher performance could be a result of either the higher rotor velocities or more diffusion downstream of the open rotor.

FIG. 3 is a diagrammatic cross section that presents a prior art diffuser augmented wind turbine (DAWT) 200 including a rotor 240 that rotates about a centerline 205 and is coaxial with an annular airfoil 212. The annular airfoil 212 has an inlet 217 and an outlet or exit plane 229. The area surrounded by the inner surface of a diffuser 215 increases in the area where diffusion occurs proximal to the exit plane 229. A control volume 233 is assumed straight and parallel to the centerline 205 of the annular airfoil 212 of the diffuser augmented fluid turbine 200. Rotor flow 231 is that flow which enters the rotor 240 within the boundaries depicted by lines 216. Diffusion occurs as the rotor flow passes over the expanding surface of the diffuser 215. The purpose of the expanding surface of the diffuser 215 is to lower the pressure at a rotor plane of the rotor 240.

The upstream wind flow velocity is depicted as 235. Rotor flow 231 that has passed through the rotor 240 and through the exit plane 229 forms exit flow 228. The streamlines 225 at the shroud exit plane 229 are assumed straight and parallel. This assumption results in ambient pressure at the shroud exit plane 229, since the pressure has to be constant in the vertical direction. Streamlines 224 represent flow that bypasses, or flows around the shroud.

Streamlines 228 represent low energy flow that passes through the rotor 240. The dashed line 233 defines the control volume used to analyze system power generation using flow conservation principles. The shroud 212 and rotor 240 forces on the fluid are represented by 218 and 220, respectively. The flow 230 exiting the control volume 233 is a result of turbine system blockage. Power 228 is extracted by the rotor. The wake velocity 228 is the same value as the rotor velocity for the control volume specified 233. The rotor flow 231 goes through no diffusion and no wake expansion downstream of the shroud exit 229. Maximum power coefficient for the turbine 200 is (0.385)*(AREAexit/AREArotor).

FIG. 4 presents a graph that plots the prior art DAWT power coefficient 157 on the vertical axis over the wake to wind velocity ratio (Vwake/Vwind) 155 along the horizontal axis. Maximum power coefficient is (0.385)*(AREAexit/AREArotor) at a wake to wind velocity ratio of 1/√{square root over (3)}. The plots depict the coefficient of power referenced to the rotor area. Six different system diffuser area ratios 159 were analyzed and are plotted. All the predicted power results fall between model 160 AREAexit/AREArotor=1, with a maximum power coefficient of 0.385 at a wake velocity ratio of 1/√{square root over (3)} and model 150 AREAexit/AREArotor=3.5, with a maximum power coefficient of 1.35 at a wake velocity ratio 1/√{square root over (3)}. That is models 152, 154, 156 and 158 fall between model 160 and model 150. The data shows that at optimum wake velocity ratio, power coefficient increases as shroud exit area increases and can exceed the values for an open rotor fluid turbine.

FIG. 5 presents a graph that plots a prior art DAWT power coefficient 137 on the vertical axis, over the wake to wind velocity ratio (Vwake/Vwind) 139 along the horizontal axis. The plot depicts the coefficient of power referenced to the shroud maximum area. All power curves for different diffuser area ratios 159 collapse into one performance line 136. Test results demonstrate that the power curves for the diffuser augmented fluid turbine collapse to the same as those for a constant area fluid turbine values presented in FIG. 2 when the power coefficient is based on maximum area. The maximum power also occurs at the same wake to wind velocity ratio of 1/√{square root over (3)}. These results demonstrate that neither higher rotor velocities, nor more flow diffusion provided solely by increasing an inner diameter of a shroud downstream of a rotor in a shrouded fluid turbine system are key to exceeding the Betz limit based on maximum flow area.

FIG. 6 illustrates downstream wake expansion with a prior art DAWT system 600. The system 600 includes a diffuser shroud 212, a rotor 240, a centerbody 251 having a centerline 253. The system exit plane is represented by bracket 230. Dash lines 227 represent different levels of wake expansion.

FIG. 7 presents the effect of wake expansion on maximum power coefficient of a shrouded fluid turbine as developed using control volume analysis. The vertical axis represents power coefficient (Cp) 353 based on maximum area and the horizontal axis represents thrust coefficient (Ct) 355. The Betz limit is represented by the dashed line 366. Dashed line 365 represents the locus of maximum performance points for each set Cpe 363. Cpe is the exit plane pressure coefficient and represents wake expansion. The lower the Cpe the larger the wake expansion downstream of the exit plane. The curves in the graph were derived using conservation of mass, momentum and energy with fixed exit plane pressure coefficients

Curve 350 on the graph has a Cpe of −0.667, curve 352 has a Cpe of −0.6, curve 354 has a Cpe of −0.5. Furthermore, it can be seen in the graph that curves 356, 357, 358, 360, 362, and 364, have Cpe levels of −0.4, −0.33, −0.3, −0.2, −0.1 and 0.0 respectively.

The lower the exit plane pressure coefficient, the larger the wake expansion as depicted in the upper curves 354, 352 and 350. In the lower region, curve 364 represents the power coefficient for a shrouded fluid turbine system with no external expansion. Curve 364 has a Cpe of 0.0 denoting no wake expansion; shrouds without wake expansion are illustrated in FIG. 1 and FIG. 3. The curve 364 depicts a maximum power coefficient (Cp) of 0.385 at a thrust coefficient of 0.667.

The results presented in FIG. 7 are consistent with the Betz limit derivation. Curve 357 has a maximum power coefficient of 16/27 with a rotor thrust coefficient of 8/9, and represents an open rotor design, having a wake expansion of 2 with a Cpe of −0.333. As shown, a Ct between 0.89 and 1.0 combined with Cpe between −0.333 and −0.70 produces a fluid turbine with the capability to exceed the Betz limit based on maximum flow area. Curves 357, 354, 352 and 350 illustrate ideal optimum performance coefficients that are between 16/27 and 0.8 and are achieved with high Ct and low Cpe.

FIG. 8 illustrates Coanda flow effect. Coanda flow produces a resultant force 489 on a propelling surface 480. Components for Coanda flow include: a high-energy flow stream 482, a large low energy medium 484, a curved surface 480, and a nozzle 488 designed to inject high-energy flow 482 parallel to the surface 480. When a thin stream of high-energy flow 482 (Coanda flow), is injected parallel to a curved surface 480 that is in contact with a large, low energy or stationary fluid 484, the high-energy stream 482 stays attached and follows the curved surface 480 even over large angular deflections. The high-energy stream 482 does not separate from the surface 480 because the low energy ambient fluid 484 cannot generate adverse pressure gradients large enough to force the high-energy flow boundary layer to separate from the surface 480. The center of pressure is the point on the curved surface 480 where all of the aerodynamic pressure field may be represented by a single force vector 489 with no moment.

FIG. 9 is a diagrammatic cross section of an example VDWT 500 of the present disclosure. FIG. 10 is a plot of the Computational Fluid Dynamics (CFD) predicted flow total pressures (or energy per unit volume) of the VDWT 500. FIG. 11 is a detail view of a plot of the CFD predicted flow velocities of the VDWT 500.

As taught herein, the inventors have discovered a fluid turbine and method of operating the same that can exceed the Betz limit. That is, the inventors have discovered that by increasing rotor energy extraction in combination with using a bypass Coanda stream to increase wake area expansion downstream of the fluid turbine advantageously results in a total kinetic energy extraction from a fluid stream that exceeds the theoretical limit of 16/27 of the kinetic energy of the fluid captured.

Referring to FIG. 9, FIG. 10 and FIG. 11. The VDWT 500 includes rotor blades 540 that are joined at a central hub or nacelle 550 and rotate about a central axis 505. An annular duct 510 is in fluid communication with the rotor blades 540 and is co-axial with the central axis 505. The annular duct 510, also referred to as a turbine shroud, includes a leading edge portion 512, also known as the inlet, that is substantially annular thus providing a relatively narrow gap between the rotor blade 540 tips and the interior surface of the leading edge 512. The turbine shroud further includes a trailing edge 516, also known as the exit of the annular duct 510. The annular duct 510 is co-axial with the rotor blades 540, and nacelle 550 about the central axis 505. A secondary shroud 520 surrounds the trailing edge 516 of the (turbine shroud) annular duct 510 and is referred to as an ejector shroud 520. The ejector shroud 520 has a leading edge 522 and a trailing edge 524 defining an exit plane 530.

In some embodiments, the length of the annular duct 510 along the central axis 505 is 30% to 70% shorter than the length of the ejector shroud 520 along the central axis 505. Those skilled in the art will appreciate that in other embodiments taught herein the length of the first annular duct relative to the central axis is 30% to 70% shorter than the length of the second annular duct relative to the central axis.

These two shrouds are used to form a Coanda nozzle 537 using the outside surface 533 of the turbine shroud 510 and the inside surface 535 of the ejector shroud 520 which allows an annulus stream of high-energy bypass flow 539 to attach and follow along the inside surface 535 of the ejector shroud 520 all the way to the exit plane 530 of the fluid turbine during full power generation. The high-energy bypass flow stream 539 is not used to energize the boundary layer. Energizing the boundary layer with bypass flow utilizes a different diffuser design and requires slot heights close to the thickness of the boundary layer(s).

The height 523 of the Coanda nozzle 537 is significantly larger than those used to energize a boundary layer, in some embodiments the height 523 of the Coanda nozzle 537 is between 5% and 25% of the length 559 of the ejector shroud. The high-energy bypass flow stream 539 follows the ejector surface, which turns outwardly between 20 degrees and 60 degrees relative to the central axis, and turns to extremely large outward angles (20 degrees to 60 degrees relative to the central axis) at the exit plane 530. The high-energy bypass flow stream 539 is a high-energy stream that lowers the exit plane 530 pressure, providing a large wake expansion 543. The high energy stream 539 bounds the expanding wake flow 543 in such a way to create an observable aerodynamic surface, or observable virtual diffuser 517.

FIG. 10 and FIG. 11, are plots of predicted flow total pressures and predicted flow velocities using CFD that illustrate the predicted flow-field through the fluid turbine 500 with a rotor under power generation. The plot represents higher total pressure gradients with lines more densely spaced than those of lower total pressure gradients, having lines less densely spaced. As in the aforementioned description of the turbine 500, the rotor blades 540 are in fluid communication with the turbine shroud 510, which is in turn in fluid communication with the ejector shroud 520.

Ambient flow 590 enters the turbine shroud 512 and passes through the rotor plane 545. Flow is accelerated by the Coanda nozzle 537 and remains in the form of a high-energy flow stream 539. The expanded wake area 543 that is downstream of the rotor plane 545 is of a lower velocity relative to the high-energy flow stream 539 as it has had kinetic energy extracted from it by the rotor blades 540. The ejector shroud 520 is designed to allow the high-energy flow stream to attach and flow to and past the exit plane 530. The high-energy flow stream 539 leaves the exit plane 530 at an outward angle between 20° and 60° with respect to the turbine central axis 505. The high-energy stream 539 decreases pressure at the exit plane 524 allowing for large wake expansion 543.

The system includes a rotor assembly 540 to extract substantially greater energy out of the flow than an open rotor or conventional shrouded fluid turbines. Extracting flow energy with the rotor assembly 540 having a coefficient of thrust (Ct) between 0.89 and 1.0 creates flow of substantially lower energy level than the bypass flow. The relatively low energy of the flow that passes through the rotor expands into the wake region 543, proximal to the inner surface 535 of the ejector shroud 520.

Low exit plane pressure is provided by a combination of rotor power extraction and the high energy flow stream 539 that forms from the Coanda nozzle. The high energy flow stream 539 creates an observable virtual diffuser 517 which bounds the expanding wake region 543. This combination provides an energy generation system with a means of extracting greater than 16/27 of the total kinetic energy from a fluid stream. In other words, with power extraction the VDWT system 500 includes the rotor assembly 540 designed with a high thrust coefficient combined with the first annular shroud 510 and the second annular shroud 520 that uses bypass flow to form an annular, high-energy flow stream 539 at the system exit plane 530 to cause large wake expansions. The large wake expansion increases the velocity and flow rate through the rotor.

The exemplary embodiments of the present disclosure advantageously operate with a bi-modal capability without requiring a change to the structure of an exemplary shrouded fluid turbine as discussed herein. When the rotor blades 540 are stationary the rotor-flow and bypass flow entering the Coanda nozzle 537 exhibit equivalent energy levels. A fluid stream proximal to the large turning angles of the system 500 exhibit flow separation 538 from the shroud surfaces 535 and 533. Flow separation over the shroud surfaces provides reduced aerodynamic loading on structural components during extreme fluid velocity events. Therefore, the VDWT concept presented herein acts significantly different with and without power extraction. In other words, the VDWT systems taught herein are bimodal. In a first mode, with power extraction the high-energy Coanda flow stream 539 at the exit plane 530 increases the wake expansion 543 and increases the velocities and flow rates through the rotor plane 545 and allows power coefficients, based on maximum system area, that are higher than the Betz limit. In a second mode, without power extraction, flow separation occurs in the system which reduces some of the aerodynamic loads.

FIG. 12 is a cross sectional view of the VDWT 500 operating in the second mode. In the second mode, flow separation 538 from the inner surface 535 of the ejector shroud 520 occurs and flow separation 557 from the inner surface of the turbine shroud 533 occurs which reduces aerodynamic loads on the VDWT at high wind conditions. As a consequence, there is reduced expansion in the wake region 543.

FIG. 13 illustrates another embodiment of the present invention. The VDWT 600 includes rotor blades 640 that are joined at a central hub 650 and rotate about a central axis 605. The hub 650 is joined to a shaft (not shown) that is co-axial with the hub and with a nacelle (not shown). A first annular duct or turbine shroud 610 is in fluid communication with the rotor blades 640 and is co-axial with the central axis 605. The turbine shroud 610 includes a leading edge portion 612, also known as the inlet, that is substantially annular thus providing a relatively narrow gap between the tips of rotor blades 640 and the interior surface of the turbine shroud 610. The turbine shroud 610 further includes a trailing edge 616, also known as the exit of the turbine shroud 610. The turbine shroud 610 is co-axial with the rotor blades 640, rotor hub 650 and nacelle about the central axis 605. A second shroud 620 surrounds the turbine shroud 610 and is referred to as an ejector shroud. The ejector shroud 620 has a leading edge 622 and a trailing edge 624 defining an exit plane 630. The leading edge 622 of the ejector shroud 620 is substantially aligned (i.e., in the same plane) with the leading edge 612 of the turbine shroud 610.

In a similar manner to the embodiment discussed in relation to FIG. 9-11, these two shrouds 610 and 620 are aligned and spaced to form a Coanda nozzle 637, using a top surface 633 of the turbine shroud 610 and a bottom surface 635 of the ejector shroud 620 which allows an annulus of high-energy bypass flow stream 639 to attach and follow along the inside surface of the ejector shroud 620 all the way to the exit plane 630 of the ejector shroud 620 and beyond during full power generation. In some embodiments the Coanda nozzle 637 has a height 623 that is between 5% and 25% of the length 659 of the second shroud 620. The high-energy bypass flow stream 639 follows the surface 635, which turns outwardly between 20 degrees and 60 degrees relative to the central axis, and turns to extremely large outward angles (20 degrees to 60 degrees relative to the central axis) at the exit plane 630. Beyond the trailing edge 624 of the ejector shroud 620 the high-energy flow stream 639 forms an observable aerodynamic virtual diffuser 617 and lower exit plane pressure which allows for greater wake expansion 643.

The VDWT 600 is also bimodal. As discussed below in relation to FIG. 12, the VDWT 600 and the VDWT 500 are bimodal. In a first mode, the VDWT 600 forms a high-energy bypass flow stream 639 which forms an observable aerodynamic virtual diffuser 617, which, in turn forms the expanded wake region 643. In a second mode of the VDWT 600, flow separation occurs from the shroud surfaces 635 which reduces some of aerodynamic loads on the VDWT 600 at high wind conditions.

As will be discussed below, other embodiments include different rotor blade locations within the turbine shroud proximal to the leading edge, proximal to the center of the turbine shroud and proximal to the trailing edge. Referring to FIG. 14, a VDWT 700 includes rotor blades 740 that are joined at a central hub 741 and rotate about a central axis 705. The hub 741 is joined to a shaft (not shown) that is co-axial with the hub and with a nacelle 750. A first shroud annular duct 710 is in fluid communication with the rotor blades 740 and is co-axial with the central axis 705. The first shroud 710, also referred to as a turbine shroud or annular duct, includes a leading edge portion 712, which defines the first shroud inlet. The first shroud 710, is substantially annular thus providing a relatively narrow gap between the tips of the rotor blades 740 and the interior surface of the first shroud 710. The first shroud 710 further includes a trailing edge 716, also known as the exit of the first shroud 710.

A second shroud 720 surrounds the trailing edge 716 of the first shroud 710 and is referred to as an ejector shroud. The second shroud 720 has a leading edge 722 and a trailing edge 724 defining an exit plane 730.

The two shrouds 710 and 720 are aligned and spaced apart to form a Coanda nozzle 737 using the top surface 733 of the first shroud 710 and the bottom surface 735 of the second ejector shroud 720. The Coanda nozzle 737 allows an annulus of high-energy bypass flow stream 739 to attach and follow the inside surface 735 of the second shroud 720 all the way to the exit plane 730 of the second shroud 720 and beyond during full power generation. In some embodiments the height 723 of the Coanda nozzle 737 is between 5% and 25% of the length 759 of the second shroud 720. The high-energy bypass flow stream 739 follows the ejector surface 735, which turns outwardly between 20 degrees and 60 degrees relative to the central axis, and turns to extremely large outward angles (20 degrees to 60 degrees relative to the central axis) at the exit plane 730 of the second shroud 720. Beyond the trailing edge of the ejector shroud 720 the high-energy flow stream 739 forms an observable aerodynamic virtual diffuser 717 and lower exit plane pressure which allows for greater wake expansion 730.

As depicted in FIG. 14, the rotor blades may be disposed at various locations along the length of the hub 741. For example, in some VDWT embodiments the leading edge of the rotor blades 740 is located at a distance 755 from the shroud leading edge 712. Substantially similar performance results are realized from rotor blades 740′ located at a distance 753 from the shroud leading edge 712; and from rotor blades 740″ located at a distance 751 from the shroud leading edge 712; from rotor blades 740′″ located at a distance 757 from the shroud leading edge 712; and from rotor blades 740″″ located at a distance 759 from the shroud leading edge 712.

Turning now to FIGS. 15 and 16, front perspective and rear perspective and detail views of an example embodiment of a VDWT 900 of the present disclosure are provided. In particular, FIG. 15 is a front perspective view of the VDWT 900 and FIG. 16 is a rear prospective view of the VDWT 900 of FIG. 15. The VDWT 900 includes a rotor assembly 942 encircled in part or completely by one or more ducts or airfoils, e.g., a turbine shroud 910 and an ejector shroud 920. The VDWT 900 can therefore be described as a ducted or a shrouded fluid turbine 900.

The VDWT 900 includes rotor blades 940 that are joined at a rotor hub 941, e.g., a rotor hub, and rotate about a central axis 905. The rotor hub 941 can be joined to a shaft (not shown) that is co-axial with the rotor hub 941 and with a nacelle 950. The nacelle 950 houses electrical generation equipment (not shown) therein. A primary airfoil, e.g., a turbine shroud 910 in the form of an annular duct, can be in fluid communication with the rotor assembly 942 and can be co-axial with the central axis 905. The turbine shroud 910 and/or the ejector shroud 920 can be secured to the nacelle 950 with one or more support beams 933 connected to an inner surface of the turbine shroud 910 and/or the ejector shroud 920. The turbine shroud 910 includes a leading edge portion 912, also known as an inlet, which can be substantially annular. The annular form of the leading edge portion 912 provides a consistent and narrow gap between the tips of the rotor blades 940 and the interior surface of the leading edge portion 912 during rotation of the rotor assembly 942. In some embodiments, the rotor assembly 942 can extend beyond the leading edge 912 and/or the trailing edge 916 of the turbine shroud 910. Although depicted as a continuous surface, in some embodiments, the turbine shroud 910 can define a faceted surface.

The VDWT 900 can include a secondary annular airfoil, e.g., an ejector shroud 920 in the form of an annular duct. The ejector shroud 920 includes a leading edge 922, e.g., an inlet end, and a trailing edge 924, e.g., an exhaust end. The leading edge 922 can be in fluid communication with the trailing edge 916, e.g., an exhaust end, of the turbine shroud 910. The ejector shroud 920 can be joined to the turbine shroud 910 at struts 906. Although depicted as a planar turbine, in some embodiments, the ejector shroud 920 can define a faceted surface. The turbine shroud 910 and the ejector shroud 920 can be co-axial with the rotor assembly 942, the rotor hub 941 and the nacelle 950 about the central axis 905. In some embodiments, the rotor assembly 942 can extend beyond the trailing edge 924 of the ejector shroud 920. The VDWT 900, the turbine shroud 910 and the ejector shroud 920 can be supported by a tower structure 902. The annular gap between an outer surface of the turbine shroud 910 and the inner surface of the ejector shroud 920 forms a Coanda nozzle 937 as described above which forms the high-energy bypass flow stream that flows along the inner surface of the ejector shroud 920 to form the observable aerodynamic virtual diffuser 917 and low exit plane pressures that drives the wake area 943 expansion beyond the outer diameter of the trailing edge 924. For example, the wake area 943 can expand to an area size greater than twice an exit area defined by the outer diameter of the trailing edge 924. Likewise the rotor assembly 942 is of a type having a high thrust coefficient as discussed above. Those skilled in the art will recognize that the wake expansion discussed above in connection with VDWT 900 is equally applicable to other embodiments of the VDWT taught herein.

As noted above, the rotor assembly 942 refers to any assembly in which blades 940 are attached to a shaft, e.g., a rotor hub 941, and are able to rotate, thereby allowing for the generation of power or energy from the fluid passing through and rotating the blades 940. In some embodiments, the rotor assembly 942 can include a propeller-like rotor and/or a stator assembly. However, it should be understood that any type of rotor assembly 942 can be utilized with the example fluid turbine 900.

The VDWT 900 is bimodal as discussed herein. In a first mode, with power extraction the high-energy Coanda flow stream 939 at the exit plane 930 increases the wake expansion 943 and increases the velocities and flow rates through the rotor plane 945 and allows power coefficients, based on maximum system area, that are higher than the Betz limit. In a second mode, without power extraction, flow separation occurs in the VDWT 900 which reduces some of the aerodynamic loads on the VDWT 900.

FIG. 17 is a right, front perspective view of an embodiment of a VDWT 1000. FIG. 18 is a left, rear, perspective view of the embodiment of FIG. 17. The VDWT 1000 includes rotor blades 1040 that are joined at a rotor assembly 1041 and rotate about a central axis 1005. The hub is joined to a shaft (not shown) that is co-axial with the rotor assembly and with the nacelle 1050. The nacelle 1050 houses electrical generation equipment 1055 (not shown). An annular primary airfoil or turbine shroud 1010 is in fluid communication with the rotor 1040 and is co-axial with the central axis 1005. The annular airfoil 1010 includes a leading edge portion 1012 also known as the inlet end and a trailing edge portion 1016 also known as the exit of the annular airfoil 1010. The turbine shroud 1010 is supported by spars 1006 that are further engaged with a secondary annular airfoil or ejector shroud 1020. The secondary annular airfoil 1020, includes a plurality of predominantly linear segments 1019. Each of the linear segments 1019 includes a leading edge 1022 and a trailing edge 1024. Each of the linear segments 1019 is in fluid communication with the trailing edge 1016 of the primary annular airfoil 1010. The VDWT 1000 is supported by a tower structure 1002.

The annular gap between an outer surface of the turbine shroud 1010 and the inner surface of the ejector shroud 1020 forms a Coanda nozzle 1037 as described above which forms the high-energy bypass flow stream 1039 that flows along the inner surface of the ejector shroud 1020 to form the observable aerodynamic virtual diffuser 1017 and low exit plane pressure that allows the wake area 1043 to expand beyond the outer diameter of the trailing edge 1024. Likewise the rotor assembly 1041 is of a type having a high thrust coefficient as discussed above.

The VDWT 1000 is bimodal as discussed herein. In a first mode, with power extraction the high-energy Coanda flow stream 1039 at the exit plane 1030 increases the wake area 1043 expansion and increases the velocities and flow rates through the rotor 1040 and allows power coefficients, based on maximum system area, that are higher than the Betz limit. In a second mode, without power extraction, flow separation occurs in the VDWT 1000 which reduces some of the aerodynamic loads on the VDWT 1000.

Each of the linear segments 1019 of the secondary annular airfoil 1020 can be pitchable relative to fixed or pitchable transitional segments or elements 1025 of the secondary annular airfoil 1020. In some embodiments, the primary annular airfoil 1010 can include a plurality of linear segments 1015. Each of the linear segments 1019 can pivot about a pivot axis 1019 that can be proximal to the leading edge 1022 of the secondary annular airfoil 1020. The pivot axis 1019 can be substantially perpendicular relative to the central axis 1005. The pitchable or pivotable segments 1019 can change the angle of attack of the secondary annular airfoil 1020 by changing the angle of attack of each segment 1019. The linear segments 1019 are movable to decrease drag on the VDWT. For example, the movable linear segments 1019 may be moved to reduce diffusion and therefore significantly wind drag values, which may help prevent damage to the VDWT in high wind conditions. The linear segments 1015 and the linear segments 1019 may be passively, mechanically, or pneumatically actuated.

A support structure, having a pivot point forward of the center of pressure 1060 and a support structure engaged with the turbine down-wind of the center of pressure 1060 provides passive yaw characteristics. The tower structure 1002 is engaged with a yaw mechanism 1036 that is in turn engaged with a substantially horizontal section 1034 that is engaged with a substantially vertical support structure 1033. The substantially vertical support structure 1033 is engaged with the nacelle 1050 and is located down-wind of the rotor and also down-wind of the center of pressure 1060 of the turbine.

FIG. 19 is a side, orthographic, detail, section view of the embodiment of FIG. 17. Each of the linear airfoil segments 1019 forms part of the ejector 1020 and each include leading the edge 1022 and the trailing edge 1024. Each of the linear airfoil segments 1019 is shown in a fully deployed state with the trailing edge 1024 above the leading edge 1022 forming chord angle 1035.

FIG. 20 illustrates an embodiment of the VDWT 1000 wherein the pitchable or articulated airfoil segments 1019 are associated with pitchable transition segments 1025. The transition segments 1025 remain deployed while the linear segments 1019 are pitched toward a streamline position. FIG. 21 illustrates the transition segments 1025 pitched toward the streamline position along with the linear segments 1019. FIG. 22 is a side, orthographic, detail section view illustrating the pitched ejector airfoil segments at the angle 1038. The pitched linear segments 1019 reduce the suction forces at the rotor station, thereby reducing aerodynamic loads at extreme wind conditions.

FIG. 23 is front perspective view of an example embodiment of a VDWT 1100 of the present disclosure are provided. In particular, VDWT 1100 has more than two ducts or shrouds. The VDWT 1100 includes a rotor assembly 1142 encircled in part or completely by one or more ducts or airfoils, e.g., a first shroud 1110 and a second shroud 1115 and a third shroud 1120. The VDWT 1100 can therefore be described as a ducted or a shrouded fluid turbine 1100.

The VDWT 1100 includes rotor blades 1140 that are joined at a rotor hub 1141, e.g., a rotor hub, and rotate about a central axis 1105. The rotor hub 1141 can be joined to a shaft (not shown) that is co-axial with the rotor hub 1141 and with a nacelle 1150. The nacelle 1150 houses electrical generation equipment (not shown) therein. A primary airfoil, e.g., a turbine shroud 1110 in the form of an annular duct, can be in fluid communication with the rotor assembly 1142 and can be co-axial with the central axis 1105. The turbine shroud 1110 includes a leading edge portion 1109, also known as an inlet, which can be substantially annular. In some embodiments the annular form of the leading edge portion 1109 provides a consistent and narrow gap between the tips of the rotor blades 1140 and the interior surface of the leading edge portion 1109 during rotation of the rotor assembly 1142. In some embodiments, as depicted in FIG. 23, the rotor assembly 1142 can extend beyond the leading edge 1109 and/or the trailing edge 1113 of the turbine shroud 1110. Although depicted as a continuous surface, in some embodiments, the turbine shroud 1110 can define a faceted surface.

The VDWT 1100 can include a secondary annular airfoil, e.g., a first ejector shroud 1115 in the form of an annular duct. The first ejector shroud 1115 includes a leading edge 1111, e.g., an inlet end, and a trailing edge e.g., an exhaust end. The leading edge 1111 can be in fluid communication with the trailing edge 1113, e.g., an exhaust end, of the turbine shroud 1110. Although depicted as a planar turbine, in some embodiments, the first ejector shroud 1115 can define a faceted surface. The turbine shroud 1110 and the first ejector shroud 1115 can be co-axial with the rotor assembly 1142, the rotor hub 1141 and the nacelle 1150 about the central axis 1105.

The VDWT 1100 can include a tertiary annular airfoil, e.g., a second ejector shroud 1120 in the form of an annular duct. The second ejector shroud 1120 includes a leading edge 1122, e.g., an inlet end, and a trailing edge 1154 e.g., an exhaust end. The leading edge 1122 can be in fluid communication with the trailing edge 1117, e.g., an exhaust end, of the first ejector shroud 1115. Although depicted as continuous surface, in some embodiments, the second ejector shroud 1120 can define a faceted surface. The turbine shroud 1110, the first ejector shroud 1115 and the second ejector shroud 1120 can be co-axial with the rotor assembly 1142, the rotor hub 1141 and the nacelle 1150 about the central axis 1105.

The VDWT 1100, the turbine shroud 1110, the first ejector shroud 1115 and the second ejector shroud 1120 can be supported by a tower structure 1102. The annular gap between an outer surface of the turbine shroud 1110 and the inner surface of the first ejector shroud 1115 forms a first Coanda nozzle 1137 a as described above which forms a first high-energy bypass flow stream 1139 that flows along the inner surface of the first ejector shroud 1115 to form a first observable aerodynamic virtual diffuser 1117 a and low exit plane pressures that drives the wake area 1143 expansion beyond the outer diameter of the trailing edge 1154. The annular gap between an outer surface of the first shroud 1115 and the inner surface of the second ejector shroud 1120 forms a second Coanda nozzle 1137 b as described above which forms a second high-energy bypass flow stream 1139 b that flows along the inner surface of the second ejector shroud 1120 to form a second observable aerodynamic virtual diffuser 1117 b and low exit plane pressures that drives the wake area 1143 expansion beyond the outer diameter of the trailing edge 1154. For example, the wake area 1143 can expand to an area size greater than twice an exit area defined by the outer diameter of the trailing edge 1154. Likewise the rotor assembly 1142 is of a type having a high thrust coefficient as discussed above. Those skilled in the art will recognize that the wake expansion discussed above in connection with VDWT 1100 is equally applicable to other embodiments of the VDWT taught herein.

The present disclosure has been described with reference to example embodiments. Modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Although the systems and methods of the present disclosure have been described with reference to example embodiments thereof, the present disclosure is not limited to such example embodiments and or implementations. Rather, the systems and methods of the present disclosure are susceptible to many implementations and applications, as will be readily apparent to persons skilled in the art from the disclosure hereof. The present disclosure expressly encompasses such modifications, enhancements and or variations of the disclosed embodiments. Since many changes could be made in the above construction and many widely different embodiments of this disclosure could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense. Additional modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure. 

1. A fluid turbine system comprising: a plurality of blades mounted to a rotor assembly, the rotor assembly having an operating thrust coefficient sufficient to create a flow region downstream of the rotor assembly that has a significantly lower energy level than a primary fluid flow; a first annular duct having an annular leading edge, an annular trailing edge, an inner surface extending between the leading edge and the trailing edge, an outer surface extending between the leading edge and the trailing edge and a central axis, the inner surface of the annular duct in fluid communication with the plurality of blades; a second annular duct having a leading edge, a trailing edge, an inner surface extending between the leading edge and the trailing edge, an outer surface extending between the leading edge and the trailing edge, and a central axis, the inner surface of the second annular duct in fluid communication with the outer surface of the first annular duct; a Coanda nozzle formed between the outer surface of the first annular duct in proximity to the leading edge of the second annular duct and the inner surface of the second annular duct in proximity to the leading edge of the second annular duct, the Coanda nozzle capable of forming a high-energy bypass fluid flow stream between the outer surface of the first annular duct and the inner surface of the second annular duct that flows along the inner surface of the second annular duct when power is extracted from the primary fluid flow by the rotor assembly, the high-energy bypass fluid flow stream flowing outwardly from the turbine system exit plane forming an observable aerodynamic virtual diffuser downstream of the exit plane of the second annular duct.
 2. The fluid turbine system of claim 1, wherein the high energy bypass flow lowers the turbine exit plane pressure allowing a wake region to expand downstream of the exit plane of the second annular shroud to a diameter greater than the outer diameter of the trailing edge of the second annular duct.
 3. The fluid turbine system of claim 2, wherein the wake region bounded by the observable aerodynamic virtual diffuser increases the volume flow rate of the primary fluid flow passing through a rotor plane of the rotor assembly.
 4. The fluid turbine system of claim 3, wherein the observable aerodynamic virtual diffuser and the volume flow rate of the primary fluid flow passing through the rotor plane increases the turbine power coefficient.
 5. The fluid turbine system of claim 1, wherein the fluid turbine system includes a first operational mode and a second operational mode.
 6. The fluid turbine system of claim 5, wherein the first operational mode occurs with power extraction when high-energy bypass flow attaches and follows the inner surface of the second duct to the exit plane and beyond during full power generation, and the second operational mode occurs when no power is extracted and any bypass flow is at the same energy level as the rotor flow resulting in boundary layer separation occurring from the inner surface of one or both of the annular ducts.
 7. The fluid turbine system of claim 1, wherein the Coanda nozzle extends circumferentially about the outer surface of the first annular surface.
 8. The fluid turbine system of claim 1, wherein an annular gap between the first annular duct and the second annular duct forms the Coanda nozzle.
 9. The fluid turbine system of claim 1, wherein the Coanda nozzle has a height of between 5% and 25% of a length of the second annular duct from the leading edge to the trailing edge along the central axis.
 10. The fluid turbine system of claim 1, wherein the rotor assembly provides a thrust coefficient of between 0.89 and 1.0 when operational.
 11. The fluid turbine system of claim 1, wherein the observable aerodynamic virtual diffuser expands a wake expansion area downstream of the exit plane of the second annular duct to an area size greater than twice an exit area defined by an outer diameter of the second annular shroud.
 12. The fluid turbine system of claim 2, wherein the rotor assembly and the expanded wake area results in a power coefficient greater than 16/27 of kinetic energy of the flow region.
 12. The fluid turbine of claim 1, wherein the central axis of the first annular duct and the central axis of the second annular duct central axis are coaxial.
 13. The fluid turbine of claim 1, wherein the inner surface on the second annular duct turns outwardly at an angle between 20 and 60 degrees with respect to the central axis.
 14. The fluid turbine of claim 1, wherein the first annular duct has a length that is 30% to 70% shorter than the second ejector annular duct.
 15. The fluid turbine of claim 1, wherein the first and second annular ducts are formed of movable faceted segments positionable to reduce diffusion.
 16. The fluid turbine of claim 15, wherein the first and second annular ducts are formed of movable faceted segments positionable to reduce aerodynamic load on the fluid turbine.
 17. The fluid turbine of claim 1, wherein the second annular duct is formed of movable faceted circumferential segments that may be actuated perpendicular to the central axis to reduce duct diffusion.
 18. The fluid turbine of claim 17, wherein the second annular duct is formed of movable faceted circumferential segments that may be actuated perpendicular to the central axis to reduce wind drag on the fluid turbine.
 19. The fluid turbine of claim 15, wherein the movable faceted segments may be passively actuated or may be mechanically or pneumatically actuated.
 20. The fluid turbine of claim 1, wherein the high-energy bypass fluid flow stream flows outwardly from the second annular duct at an angle of between about 20° and about 60° relative to the central axis.
 21. A method of increasing energy extraction from a fluid stream, comprising: forming a high-energy bypass flow between an annular gap formed between an outer surface of a first annular duct and an inner surface of a second annular duct downstream of the first annular duct, forming an observable aerodynamic virtual diffuser from the high-energy bypass flow downstream of an exit plane of the second annular duct, forming with the low exit plane pressure provided from the high-energy bypass flow a wake area larger than an area defined by an exit plane of the second annular duct, and extracting energy from the flow stream with a rotor assembly having a thrust coefficient of between 0.89 and 1.0.
 22. A ducted fluid turbine comprising: a first annular ducted shroud, a second annular ducted shroud disposed downstream of the first annular ducted shroud, an annular gap formed between a top trailing edge surface of the first annular shroud and a leading edge of the second annular shroud, the annular gap having a height sufficient to form a fluid jet along an inner surface of the second annular shroud to form an observable aerodynamic virtual diffuser downstream of an exit plane of the second annular shroud, and a rotor assembly disposed in the first annular shroud having a thrust coefficient of between 0.89 and 1.0.
 23. The ducted fluid turbine of claim 22, wherein the high energy bypass flow lowers the turbine exit plane pressure allowing the wake to expand downstream of the exit plane of the second annular shroud to a diameter greater than the outer diameter of the trailing edge of the second annular duct.
 24. The ducted fluid turbine of claim 23, wherein the wake region bounded by an observable aerodynamic virtual diffuser increases the volume flow rate of the primary fluid flow passing through a rotor plane of the rotor assembly.
 25. The ducted fluid turbine of claim 24, wherein the observable aerodynamic virtual diffuser and the volume flow rate of the primary fluid flow passing through the rotor plane increases the turbine power coefficient.
 26. The ducted fluid turbine of claim 22, wherein the ducted fluid turbine includes a first operational mode and a second operational mode.
 27. The ducted fluid turbine of claim 26, wherein the first operational mode occurs with power extraction when high-energy bypass flow attaches and follows the inner surface of the second shroud to the exit plane and beyond during full power generation, and the second operational mode occurs when no power is extracted and any bypass flow is at the same energy level as the rotor flow resulting in boundary layer separation occurring from the inner surface of one or both of the annular ducts.
 28. The ducted fluid turbine of claim 22, wherein the annular gap forms a Coanda nozzle.
 29. The ducted fluid turbine of claim 28, wherein the Coanda nozzle has a height of between 5% and 25% of a length of the second shroud from the leading edge to the trailing edge along a central axis of the second shroud.
 30. The ducted fluid turbine of claim 22, wherein the rotor assembly and the expanded wake area results in a power coefficient greater than 16/27 of kinetic energy of fluid flowing through the first shroud.
 31. The ducted fluid turbine of claim 22, wherein a central axis of the first shroud and a central axis of the second shroud are coaxial.
 32. The ducted fluid turbine of claim 22, wherein an inner surface on the second shroud turns outwardly at an angle between 20 and 60 degrees with respect to a central axis.
 33. The ducted fluid turbine of claim 22, wherein the first shroud has a length that is 30% to 70% shorter than a length of the second shroud along a central axis.
 34. The ducted fluid turbine of claim 22, wherein the first and second shrouds are formed of movable faceted segments positionable to reduce diffusion.
 35. The ducted fluid turbine of claim 34, wherein the first and second shrouds are formed of movable faceted segments positionable to reduce aerodynamic load on the fluid turbine.
 36. The ducted fluid turbine of claim 22, wherein the second shroud is formed of movable faceted circumferential segments that may be actuated perpendicular to a central axis to reduce duct diffusion.
 37. The ducted fluid turbine of claim 36, wherein the second shroud is formed of movable faceted circumferential segments that may be actuated perpendicular to a central axis to reduce wind drag on the fluid turbine.
 38. The ducted fluid turbine of claim 34, wherein the movable faceted segments may be passively actuated or may be mechanically or pneumatically actuated.
 39. The ducted fluid turbine of claim 22 further comprising, a third annular ducted shroud disposed downstream of the second annular ducted shroud.
 40. The ducted fluid turbine of claim 39 further comprising, another annular gap formed between a top trailing edge surface of the second annular shroud and a leading edge of the third annular shroud, the annular gap having a height sufficient to form another fluid jet along an inner surface of the third annular shroud to form another observable aerodynamic virtual diffuser downstream of an exit plane of the third annular shroud. 